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Recapped Macintosh Classic with checkered screen

goerz

Member
Hello, I recapped a Classic with capacitor damage on the analog board a few weeks ago (logic and analog board). Everything worked fine for a while, but today this pattern appeared on my monitor. I managed to narrow down the problem to the analog board (I swapped it with another one and the Mac works fine). Since I already replaced all the capacitors (including safety capacitors, the TDA4605 (IP1) chip, the phototransistor (CY17G-3), Q2 mosfet (IRFBC40), and diodes DP3 and DP4, what else could I do? Could it be a bad flyback? Thanks.



IMG_20220604_115648.jpg
 

joshc

Well-known member
It's not a bad flyback.

What voltages do you get? Depending on which rail is weak/out of spec, this could lead to what you need to look at.

The TDA 4605 might need replacing.
 

goerz

Member
It's not a bad flyback.

What voltages do you get? Depending on which rail is weak/out of spec, this could lead to what you need to look at.

The TDA 4605 might need replacing.
I get 4.5v on the external floppy port and 11.78 on the hard drive connector. I have already replaced the TDA 4605.
 

3lectr1cPPC

Well-known member
I’m no expert in power supplies, but that seems a tad bit low to me, especially on the 5v rail. I believe these analog boards have voltage adjustments that may need to be messed with after a recap to get it closer to spec. Not sure where they are because I don’t own one,but adjusting those may be worth trying, I could be wrong though.
 

dochilli

Well-known member
You need 4.85 V. With 4.5 you get a checkerboard. You could try to turn pp1, but check the 12 V, when you turn the poti!
 

goerz

Member
You need 4.85 V. With 4.5 you get a checkerboard. You could try to turn pp1, but check the 12 V, when you turn the poti!
Thank you: I turned pp1 all the way up, and I got only 4.6v. Moreover, the screen wobbles and the pattern changed to what you can see below.


IMG_20220604_161824.jpg
 

goerz

Member
So I fiddled with pp1 and I managed to get 5.1v: the issue is solved, the Classic now boots. However the screen "trembles" for a while, then it stabilizes. I need to perform more tests... Thanks.
 

goerz

Member
Did you check the 12V, when you have 5.1 V?
It's 12.68v. But I was too quick to call the issue solved: the screen reverted back to a wobbling stripe pattern, with no startup sound, once I switched on the computer after a few hours. However, after four or five minutes it warms up, then disk icon appears and it boots. Now I tested for an hour or so and the screen is fine, no wobbling or trembling. I need to check voltages in the warm-up stage, but I guess it slowly reaches 5.1v. What could be the cause of this behavior?
 

goerz

Member
I'd just add more info, in case someone can help me. As I wrote, the voltage starts at 4.4, then it slowly increases. When it gets to 4.85 the pattern disappears and the computer boots, however, the image wobbles visibly. In about 5 minutes the voltage reaches 5.15v and then it stabilizes. At this point, the image on the screen is clear and stable, without tremors. I tested the Mac for a few hours and it appears stable. The problem appears again whenever the analog board is cold, i.e., a few hours after the last time it was turned. on.
 

Jamieson

Well-known member
I have a Classic II with a similar issue that I'm also trying to track down.
Let the voltages stabilize, then turn off the power for a few seconds. Turn it back on again. Does it stabilize any faster this time?
Did you also replace the large high voltage cap(s) 220uf 250V?
Does anyone know if this power supply design has a NTC/thermistor type thing on the input to limit startup surge?
 

Jamieson

Well-known member
There is an NTC device (RP2) near the AC input. The schematic shows it should have a resistance of 20 ohms when cold, or something like that. It's designed to heat up and as it does the resistance drops. Check it for damage. You can probably also measure its cold resistance in circuit.
 
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goerz

Member
There is an NTC device (RP2) near the AC input. The schematic shows it should have a resistance of 20 ohms when cold, or something like that. It's designed to heat up and as it does the resistance drops. Check it for damage. You can probably also measure its cold resistance in circuit.
Thank you for your reply: yes, the voltage stabilizes much faster once the computer is warmed-up, even after switching it off. I did replace all the capacitors, including the high-voltage ones (btw, it's a 220v board). I'll check the NTC device you mentioned.
 

goerz

Member
Maybe the pp1 is bad?

I did not have the checkerboard but a wobbly screen with low voltages after recap.

Have a look at this thread:
https://68kmla.org/bb/index.php?threads/mac-classic-with-a-wobbly-screen-after-recap.37909/
I read the thread you linked, PP1 could indeed be the culprit. However, it kind of works, meaning that it does adjust voltage as desired. The maximum voltage I reach once it is stabilized is affected by pp1. I'll try to clean it and in the meantime I'll order a replacement.
 

Jamieson

Well-known member
I'll check my Classic II power rails, I suspect they are doing the same thing, but on the timescale of about 30 seconds. I can hear the fan slowly increase speed at powerup so I know the 12V rail is doing something similar.

Maybe one thing to try, to see if it is thermal related, is heat up a small section of the analog board with a hot air gun. If you hit a component with heat and it immediately jumps to the proper levels then you found the troublemaker.
 

goerz

Member
So I checked the resistance of the RP2 thermistor and it's exactly 20 ohms when cold, so it can't be the culprit. I replaced the PP1 voltage regulator and for a moment I thought I solved the problem, but then I realized that the board was probably still heated because of my soldering. In fact, after a few hours I tried to boot it again and the voltage was still low, although the situation seems to have improved: instead of starting at 4.3v, it now starts at 4.8v, so the Mac boots right away. The screen however continues to tremble and wobble for a few minutes, until the voltage stabilizes. Maybe PP1 has no effect at all and it's just because of the increased room temperature (summer is coming quickly). Maybe I could try putting the analog board in the fridge for a few hours...
 
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